The disclosures herein relate generally to flexible circuits and more particularly to flexible circuits with covercoat layers formed to have a controlled placement.
Flexible circuits generally include a pattern of conductive traces that are supported on a base substrate such as a layer of dielectric material. The conductive traces typically have a copper core plated with a corrosion resistant material such as gold. Polyimide is a common base substrate. U.S. Pat. Nos. 4,987,100; 5,227,008; 5,334,487; 5,557,844 and 5,680,701 disclose processes for fabricating printed circuits having a flexible polymeric base substrate such as polyimide. U.S. Pat. Nos. 3,660,726; 4,029,845; 4,526,835; and 5,806,177 disclose processes for fabricating printed circuits having a generally rigid base substrate such as a glass reinforced epoxy composite.
Electronic packages, medical devices, hard disk drive suspensions and ink jet printer pens are common applications for flexible circuits. Flexible circuits offer attributes such as fine pitch traces, complex circuit designs and flexibility. Depending on the design and specific application, a flexible circuit may have an opening formed in the base substrate. One or more of the conductive traces may include a lead that extends in a cantilevered manner from an edge of the opening. The leads may also be formed in a manner in which they the span across the opening.
In some applications, flexible circuits may be exposed to an aggressive environment. Unprotected conductive traces and the interface between the conductive traces and the base substrate are two areas susceptible to being affected by adverse environmental conditions such as exposure to corrosive fluids. Exposing unprotected conductive traces to adverse environmental conditions typically leads to the traces corroding or delaminating from the base substrate.
The flexible circuit is typically attached to a rigid structure such as a stiffening member or the body of a printer pen. The leads may be interconnected to an electronic device carried by the rigid structure or to an electronic device that is attached directly to the base substrate of the flexible circuit. Typically, the side of the base substrate carrying the conductive traces is attached to the rigid structure. An encapsulant is typically applied over the leads to provide a degree of protection from adverse environmental conditions.
A covercoat layer is sometimes formed over the conductive traces to prevent the traces from being exposed to adverse environmental conditions. The covercoat layer is often a photoimageable material that is patterned using UV light and a photomask. Due to limitations in conventional methods of forming the covercoat layer, the resulting covercoat layer does not have a uniform and controlled thickness or a well-defined pattern. In some areas, the thickness of the covercoat can be insufficient to provided adequate protection against adverse environmental conditions. This is often the case with circuits having portions of the traces beyond an edge of the substrate (called leads).
An encapsulant is often applied to protect the leads. Depending on the type of device attached to the leads, the encapsulant may also be used to protect the device (e.g. a bare semiconductor chip). Because of the orientation of the flexible circuit, it is easy to encapsulate the outward facing side of the conductive traces. However, reliably encapsulating the inward facing side of the circuit is difficult. Air pockets formed adjacent to the leads during encapsulation can serve as pathways for contaminants to reach the traces. Over time, traces having an insufficient thickness of covercoat material can be attacked by contaminants, causing the flexible circuit, the attached device, or both, to fail.
Accordingly, a need has arisen for a base substrate having a covercoat formed thereon in which the shortcomings of previous techniques and constructions are overcome.
It has now been discovered that a photoimageable covercoat can be formed which has controlled placement beyond the base substrate and onto the lead portion of the conductors. Use of a covercoat with such improved placement characteristics also helps to alleviate problems caused by air pockets in the encapsulant by providing an additional layer of protection. Further, a photoimageable covercoat can be formed which can be coated to a controlled thickness beyond the edge of the base substrate to assist in corrosion protection, and the like.
The invention provides a printed circuit having improved resistance to adverse environmental conditions. The printed circuit includes a base substrate having conductors formed thereon. A portion of some or all of the conductors form traces and a portion form leads extending from an edge of the base substrate. A photoimageable covercoat layer is formed on the base substrate including a lead portion formed on the leads extending beyond the substrate at one or more edges. The lead portion of the photoimageable covercoat layer has a controlled and substantially uniform placement, i.e., a predetermined and substantially controlled offset distance.
The lead portion of the photoimageable covercoat may be a direct extension of a trace portion of the photoimageable covercoat, i.e., it may be a continuous coating, or it may be a completely detached coating.
The base substrate and printed circuits formed therefrom also have a nonconductive portion, i.e., that part of the substrate which is uncovered by a conductor. Some or all of this portion may also be covered by the covercoat.
In a preferred embodiment, the photoimageable covercoat also has a controlled thickness beyond the edge of the base substrate, and covers at least a portion of the upper surface and the lower surface of the leads.
A principal advantage of the embodiments presented herein is that the leads can be encapsulated with a uniform layer of covercoat material to reduce the potential of the leads being attacked by contaminants. A uniform layer of covercoat can be formed on the leads to encapsulate and protect the leads.
Another principal advantage is the ability, when using a substrate having an interior opening, to form three-dimensional features within the opening on the lead portion of the covercoat; a conductor having a lead extending into such opening need not be present to form such a feature.
As used herein, these terms have the following meanings:
1. The term xe2x80x9ctracexe2x80x9d refers to that portion of the conductor(s) which is supported on a base substrate.
2. The term xe2x80x9cleadxe2x80x9d refers to that portion of the conductor(s) which is unsupported by the base substrate, e.g., a conductor extending beyond an edge of the polyimide substrate.
3. The term xe2x80x9clead portionxe2x80x9d when referring to the photoimageable covercoat refers to the portion of the covercoat which extends beyond an edge of the polyimide substrate. Note: this term is used for the portion of the covercoat extending beyond an edge of the substrate whether or not a conductor lead also extends therefrom.
4. The term xe2x80x9ctrace portionxe2x80x9d, when referring to the photoimageable covercoat refers to the portion of the covercoat which coats at least one trace portion of the conductor on the base substrate.
5. The term xe2x80x9cUVxe2x80x9d means ultraviolet and refers to radiation from a source having wavelengths of from about 100 to about 4500 Angstroms.
6. The term xe2x80x9coffset distancexe2x80x9d means the distance which the photoimageable covercoat extends beyond the edge of the base substrate.
7. The term xe2x80x9cline and space patternxe2x80x9d means a photomask is used that allows only certain portions of the covercoat surface to be exposed to UV, while other areas receive no UV exposure.
8. The term xe2x80x9cfloodxe2x80x9d or xe2x80x9cflood exposexe2x80x9d refers to a UV exposure in which at least one entire covercoat surface is exposed to UV.
9. The term xe2x80x9cinterior edgexe2x80x9d refers to an edge originally facing an opening formed in the interior of a base substrate during processing. Such an edge may become an exterior edge on the final circuit after singulation.